Probing the Rotary Motions of Immobilized Photon-Driven Molecular Actuators Using Normal Incidence Cavity Ring-Down Spectroscopy

Abstract

A family of substituted 9-(2,2,2-triphenylethylidine)fluorene molecules were studied as surface immobilized light driven molecular actuators. 2-Nitro-, 2-amino-, 2-cyano-, 2-iodo-, and 2-t-butyl-9-(2,2,2-triphenylethylidene)fluorene in addition to the unsubstitutedmolecule, were immobilized in a poly(methyl methacrylate) (PMMA) matrix adsorbed on a fused silica substrate at room temperature. UV irradiation causes reversible spatial reorientation of the angular distribution of the dibenzofulvene rotor moieties resulting fromphotoisomerization of the exocyclic double bond. This reorientation was detected by multichannel polarization-resolved normal incidence cavity ring-down spectroscopy (NICRDS)which probed the reorientation dynamics when the actuator molecule was driven by linearly polarized light. Polarized "drive" irradiation creates a "hole" in the angular distribution of the molecular transition dipoles. Changing the polarization of the drive beam refills the hole creating a new hole parallel to the polarization direction of the drive beam. The photo-reorientation kinetics measurements of the hole burning and refilling were fitted to a stochastic model to obtain a photo-reorientation quantum yield. Ideally, the photo-reorientation quantum yield is equal to the photoisomerization quantum yield, but the values obtained were smaller than the photoisomerization quantum yields measured in organic solution. This may be dueto the PMMA different environment, dipolar intermolecular forces, or greater steric hindrance. In the case of an unsubstituted actuator, the technique proves to be a novel method for indirectly measuring the photoisomerization quantum yield of two indistinguishable isomers, by measuring photo-reorientation quantum yield .